1. Field of the Invention
The present invention relates to a protection circuit for protecting a semiconductor integrated circuit device and its internal circuits when an overvoltage or a surge of any kind is applied to a terminal of the semiconductor integrated circuit device. The present invention relates also to a semiconductor integrated circuit device provided with such an overvoltage detection circuit.
2. Description of the Prior Art
As electronic appliances employing semiconductor integrated circuit devices become increasingly varied, and their uses become increasingly varied, more and more cases are reported in which an overvoltage in the form of noise, a surge, static electricity, or in any other form flows directly or indirectly, for example through a human body, into an electric circuit, destroying an electronic appliance or inflicting serious damage thereon. Even at manufacturing sites of electronic appliances, the measures taken against such hazards are not sufficient to satisfactorily suppress rejection rates.
For this reason, in an electronic circuit or semiconductor integrated circuit device, a terminal thereof that is likely to be exposed to an overvoltage is provided with a protection circuit to prevent an overvoltage from being applied to a functional circuit provided inside.
FIGS. 6 and 7 show conventional examples of such overvoltage protection circuits. FIG. 6 shows conventional overvoltage protection circuits. In FIG. 6, reference numeral 10 represents a semiconductor integrated circuit device having terminals “a” to “m”, to each of which is connected a protection circuit composed of n Zener diodes ZD1 to ZD(n) connected together in series, with the anode of the Zener diode at the other end grounded. Though not illustrated, to each terminal is connected, in addition to one end of such a protection circuit, one end of an internal functional circuit.
In this circuit configuration, an internal circuit of the semiconductor integrated circuit device 10 is protected from a voltage higher than a predetermined voltage by setting a protection voltage that is determined by the characteristics of Zener diodes, namely, their Zener voltage and how many of them are connected together. Specifically, to protect an internal circuit of the semiconductor integrated circuit device 10 from an overvoltage higher than 100 V, for example, 10 Zener diodes that each have a Zener voltage of 10 V and that are connected together in series are connected to each of the terminals “a” to “m,” so that, even when an overvoltage higher than 100 V is applied to any of the terminals “a” to “m,” a reverse current flows through the Zener diodes ZD1 to ZD(n) to ground, and thus no overvoltage higher than 100 V flows into the unillustrated internal circuit. On the other hand, the normal signal voltages that are fed to the internal circuit via the terminals “a” to “m” are all lower than the protection voltage so set. Thus, for example, when a 5V signal voltage is fed to the terminal “a,” no reverse current flows through the Zener diodes, and therefore the signal voltage is properly fed to the internal circuit.
FIG. 7 shows conventional overvoltage protection circuits incorporating overvoltage detection circuits. Here, such circuit elements as have the same functions as in FIG. 6 are identified with the same reference symbols. Reference numeral 11 represents a semiconductor integrated circuit device having terminals “a” to “m.” To the terminal “a” is connected a protection circuit, similar to those shown in FIG. 6, composed of n Zener diodes ZD1 to ZD(n) connected together in series, with the anode of the Zener diode at the other end grounded through a resistor R1, of which one and the other ends are connected to the base and emitter, respectively, of an NPN-type transistor Qa. Though not illustrated, the collector of the transistor Qa is connected, for example, to an internal circuit of the semiconductor integrated circuit device. The circuits for the other terminals are configured likewise.
In this configuration, how the internal circuit is protected from an overvoltage by the Zener diodes is the same as in FIG. 6, and therefore no explanation of it will be repeated. Now, the overvoltage detection circuit composed of the resistor R1 and the transistor Qa will be described. When an overvoltage causes an overcurrent I1 to flow through the Zener diodes ZD1 to ZD(n) and through the resistor R1 to ground, a voltage drop of I1×R1 occurs across the resistor R1, turning the transistor Qa on. Accordingly, by connecting the collector of this transistor Qa to the unillustrated internal circuit of the semiconductor integrated circuit device, it is possible to extract an overvoltage detection signal SA to disable the internal circuit momentarily or until the entire circuit is reset.
As described above, according to the conventional technique, many Zener diodes need to be connected to each terminal that is likely to be exposed to an overvoltage. This not only makes a semiconductor integrated circuit device unduly large, but also increases the number of fabrication steps and thus costs. Moreover, variations in the characteristics of the circuit elements used cause variations in the protection voltage from one terminal to another.